A mobile crane in the form of a truck mounted crane typically includes a transport chassis and a superstructure coupled to the transport chassis. The superstructure typically includes an extendable boom. In transport, the crane is supported by the chassis on its axles and tires. At times, the crane needs to be stabilized beyond what can be provided while resting on the tires of the transport chassis. In order to provide stability and support of the crane during lifting operations, it is well known to provide the chassis with an outrigger system. An outrigger system will normally include at least two (often four or more) outriggers for supporting the crane when the crane is located in a position at which it will perform lifting tasks.
FIG. 1 illustrates a conventional type of outrigger system 120 commonly referred to as an A-frame outrigger. This outrigger system 120 includes telescoping legs 122 having an upper segment 126 and a lower segment 128. The upper segment 126 is pivotally attached to a support 114 such as the crane superstructure. The lower segment 128 telescopes from the upper segment 126 and interacts with a base surface to support the mobile crane. An intermediate link 124 is pivotably attached to the support 114 at a first end 130 and to the lower segment 128 at a second end 132. A linear actuator, such as a hydraulic cylinder, selectively extends and retracts the lower segment 128 relative to the upper segment 126. The linear actuator may be internal to the upper segment 126 and lower segment 128 and is not visible in FIG. 1.
When the telescoping leg 122 is retracted as shown on the right hand side of FIG. 1, the intermediate link 124 is nearly vertical and the telescoping leg 122 is held close to the mobile crane. As the lower portion 128 of the telescoping leg 122 is extended, the second end 132 of the intermediate link 124 moves with the lower portion 128, rotating the intermediate link 124 outward. The rotating intermediate link 124 pushes the telescoping leg 122 outward, angling the telescoping leg 122, as shown on the left hand side of FIG. 1. Together, the telescoping leg 122, the intermediate link 124, and the support 114 form a fixed triangle. The span of the A-frame outrigger 120 is fixed, dependent upon the geometry of the outrigger.
Another type of outrigger is known in the art as an out-and-down outrigger. An out-and-down outrigger typically includes a telescoping beam that may extended outward from or retracted toward a crane chassis in a horizontal direction (i.e., parallel to a support surface) and a jack extendable from or retractable toward the beam in a vertical direction. Such an outrigger is shown, for example, in U.S. Pat. No. 4,394,912, to Epps et al., the disclosure of which is incorporated herein by reference in its entirety. In an out-and-down outrigger, separate actuators move outrigger pads in/out by actuation of the telescoping beam, and up/down by actuation of the jacks, respectively. An out-and-down outrigger may be advantageous in that they allow the outrigger span to be adjusted independent of the vertical placement of the pad. That is, a vertical position of the jack (or pad) may be adjusted independently of a horizontal position of the beam, and vice versa. However, out-and-down outriggers are necessarily larger than an A-frame outrigger, since they act as horizontal beams supporting the crane, whereas the A-frame outrigger supports the crane nearly in line with the telescoping leg. As such, A-frame outriggers may be advantageous compared to other outriggers in that they are significantly less expensive, they are space efficient, and they require only a single linear actuator. However, as described above, conventional A-frame outriggers are limited to fully extended and fully retracted positions, and thus, do not allow for intermediate positioning for support and stabilization of the crane at multiple extended positions.
Accordingly, it is desirable to provide an outrigger that combines the low cost and reduced complexity of an A-frame outrigger, while allowing the span of the outrigger to be adjustable like an out-and-down outrigger.